This is a multcenter, randomized, double-blinded, placebo-controlled clinical trial to determine whether a recombinant human interferon beta product, interferon beta-1a (AVONEX), is beneficial in delaying the onset of a new neurological event in subjects who have experienced a first and recent onset of a demyelinating event involving the optic nerve, spinal cord or brainstem and who are at high risk for multiple sclerosis based on the presence of multiple brain magnetic resonance imaging (MRI) signal abnormalities.